Project Abstract Parsonage-Turner Syndrome (PTS) is characterized by marked weakness in the distribution of one or more upper extremity peripheral nerves and typically ensues following an intense pain prodrome. PTS likely arises from an immune-mediated, inflammatory response to a stressful trigger (e.g. exertion, viral infection), but its specific pathogenesis is unknown. There is a significant burden of disability in PTS, with up to two-thirds of patients reporting chronic pain and/or persistent weakness. Recent electrodiagnostic and imaging research by our group and others has localized PTS lesions to brachial plexus branches and more peripheral nerves. Using high-resolution MRI and ultrasound (US) techniques, we routinely identify intrinsic, ?hourglass? constrictions (HGCs) of involved nerves or nerve fascicles and detect denervation changes in muscles they supply. There is no currently accepted treatment or published randomized, controlled trial evaluating immunomodulatory therapy for PTS, with only a few retrospective reports of improvement with steroids and/or intravenous immunoglobulin (IVIg). This is attributed to the absence of a validated disease biomarker for PTS, with the diagnosis currently hinging on the clinical presentation. In recalcitrant cases, surgical microneurolysis of the HGC is sometimes recommended. Precise, pre-operative imaging localization of HGCs at our institution has facilitated targeted microneurolysis in recalcitrant cases, with positive early results. Pre- operative localization reduces potential morbidity associated with a time-intensive, extensive nerve exploration and the possibility of missing pathology. This research plan aims to understand the natural course of imaging and serologic findings in PTS and to determine if they can serve as non-invasive markers of motor and overall functional recovery. Subjects will undergo MRI and US of their upper extremities, and HGC findings will be correlated with pain scores, muscle strength, electrodiagnostic testing, serologic inflammatory markers, and patient reported outcome measures, all obtained upon initial evaluation and at follow-up intervals of 3 and 6 months. We will also evaluate MRI as a potential quantitative biomarker of muscle denervation that can be used to predict patient outcomes. Validated disease biomarkers would facilitate a subsequent randomized trial comprising medical therapy (e.g. steroids, IVIg) or surgical microneurolysis of HGCs. Early identification of PTS cases involving nerves that have limited or no potential for reinnervation (because of constrictions) may ultimately increase the success of microneurolysis.